Prior image-forming apparatus have included a PWM circuit that outputs a pulse width modulation (PWM) signal, and a high voltage generation unit that applies electricity corresponding to the pulse width of the PWM signal to the electric load. The power supply device compares the current value of electric power supplied to the electric load with a target power value. If the current power value does not accord with the target power value, the pulse width of the PWM signal is increased or decreased by a predetermined amount stepwise by changing the counter value of the up-down counter until both values accord with each other, thus bringing the current power value to the target power value.
Under the above-described pulse width control for increasing or decreasing the pulse width of the PWM signal upon detection of the difference between the actual power value and the target power value, the follow-up delay inevitably occurs in the entire control system. The change in the power output from the high-voltage generation circuit may occur behind the pulse width control operation. Under the generally employed power control mode, the pulse width of the PWM signal is fixed to a constant value when the power value at the electric load reaches the target power value at the pulse width adjustment. Then the power value further changes owing to the follow-up delay, that is, overshoots, thus failing to execute a stable power supply control. The overshoot is a temporal phenomenon caused by the follow-up delay, and the power value will be brought into the steady state as passage of time. Therefore, the power control system may be structured to be held stand-by state until the power value is brought into the steady state after changing the pulse. In the aforementioned structure, however, as the time period for stand-by is required, quick power supply control cannot be executed. This may deteriorate quality of the image on the image-forming apparatus.
Thus, there is a need in the art for a power supply device that allows quick and stable power supply control to an electric load, and an image-forming apparatus provided with the power supply device.